Transformer Objective Type Question Answers

Figure 5

41. The impedance seen by the primary in Fig. 5 is 

(i) 5 Ω
(ii) 25 Ω
(iii) 625 Ω
(iv) none of the above

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42. The primary current Ip in Fig. 5 is 

(i) 2 A
(ii) 0.04 A
(iii) 0.004 A
(iv) none of the above

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43. The voltage across 40 KΩ resistor in Fig. 5 is  

(i) 1000 V
(ii) 500 V
(iii) 800 V
(iv) none of the above

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44. The voltage across 20 KΩ capacitor in Fig. 5 is  

(i) 200 V
(ii) 400 V
(iii) 1000 V
(iv) none of the above

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45. The secondary current Is in Fig. 5 is 

(i) 1.25 A
(ii) 50 A
(iii) 0.02 A
(iv) none of the above

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46. Cores of large transformers are built upto nearly circular cross-section in order to reduce 

(i) leakage reactance
(ii) iron losses
(iii) eddy current loss
(iv) copper loss

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47. The core-type transformer provides 

(i) much longer magnetic path
(ii) shorter magnetic path
(iii) lesser average length per turn
(iv) none of the above

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48. The core-type transformer is generally suitable for 

(i) high voltage and small output
(ii) high voltage and high output
(iii) low voltage and high output
(iv) none of the above

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49. Transformers having ratings less than 5 k VA are generally  

(i) oil cooled
(ii) natural air cooled
(iii) water cooled
(iv) none of the above

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50. The transformer that should never have the secondary open-circuited when primary is energised is 

(i) power transformer
(ii) voltage transformer
(ii) auto transformer
(iv) current transformer

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51. The temperature rise of a transformer is directly proportional to 

(i) apparent power
(ii) reactive power
(iii) leakage reactance
(iv) none of the above

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52. In an auto transformer, the primary and secondary are ………. coupled.

(i) only magnetically
(ii) only electrically
(iii) magnetically as well as electrically
(iv) none of the above

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Figure 6

53. The impedance reflected back to the source in Fig. 6 is 

(i) 11.2 Ω
(ii) 22.2 Ω
(iii) 25 Ω
(iv) 47.8 Ω

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54. The secondary load impedance of a step-up transformer is Zs The primary impedance will be

(i) equal to Zs
(ii) greater than Zs
(iii) less than Zs
(iv) none of the above

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55. The steel used for transformer core has

(i) high silicon content
(ii) high permeability
(iii) low hysteresis loss
(iv) all of the above

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56. In a transformer, leakage flux

(i) helps in transfer of energy
(ii) is negligible at full-load
(iii) is minimised by interleaving the primary and secondary windings.
(iv) produces mutually induced e.m.f.

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57. Short-circuit test of a transformer helps us to find its

(i) iron loss
(ii) full-load Cu loss
(iii) Cu loss at no load
(iv) none of the above

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58. A two-winding transformer operates at maximum efficiency when its

(i) hysteresis loss equals eddy current loss
(ii) Cu loss equals iron loss
(iii) primary resistance equals secondary resistance
(iv) voltage regulation is minimum.

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59. The all-day efficiency of a transformer is also called its

(i) energy efficiency
(ii) power efficiency
(iii) current efficiency
(iv) none of the above

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60. The short-circuit test helps us to find

(i) Cu loss at any desired load
(ii) iron loss at no load
(iii) Cu loss at no load
(iv) none of the above

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61. The power efficiency of a lighting transformer is always ………. its all-day efficiency.

(i) equal to
(ii) higher than
(iii) less than
(iv) none of the above

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62. With a load of leading power factor, the full load secondary voltage of a transformer is ………  its no-load voltage.

(i) equal to
(ii) less than
(iii) greater than
(iv) none of the above

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63. What is the approximate efficiency of large transformers ?

(i) 65 %
(ii) 75 %
(ii) 85 %
(iv) 95 %

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64. An isolation transformer has primary to secondary turns ratio of

(i) 1 : 1
(ii) 1 : 2
(iii) 2 : 1
(iv) can be any ratio

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65. The load on a transformer is 2 ∠0o Ω or  2 ∠20o Ω. The efficiency of transformer will be

(i) greater for a load of 2 ∠0o Ω
(ii) greater for a load of 2 ∠20o Ω
(iii) the same for two loads
(iv) data insufficient

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